CREATE TABLE PEOPLE (
    ID_P      INTEGER NOT NULL,
    ID_EXTP   INTEGER,
    ...
    CATEGORY  VARCHAR( 30 ),
    N_PRIV    INTEGER,
    ...
);

ALTER TABLE PEOPLE ADD CONSTRAINT UNQ_NPRIV UNIQUE (N_PRIV);

CREATE TRIGGER PEOPLE_BI0 FOR PEOPLE
ACTIVE BEFORE INSERT POSITION  0 
AS
begin
  if (New.category is not NULL) then
   /*если новое значение "категория" есть, выбираем макс.значение
  "Льготн. очереди" добавляем 1 и записываем*/ 
  UPDATE PEOPLE SET new.n_priv = (select max(N_PRIV) from PEOPLE)+ 1 ;
   /* Trigger text */ 
end

CREATE TRIGGER PEOPLE_BU0 FOR PEOPLE
ACTIVE BEFORE UPDATE POSITION  0 
AS
begin
  if (new.category is null) then
   /*если поле "категория" пустое, поле льг.очереди делаем пустым*/ 
  update people set New.n_priv = NULL;
   /* Trigger text */ 
end